The present invention relates to an image processing apparatus wherein images are formed on an image carrier by providing image data in which original image signals were obtained by photoelectrically converting the original image with a solid-state image sensor was subjected to various image processes, and more particularly to an image processing apparatus wherein an increase in writing speed is attempted.
As a method to realize an increase in the speed of the image processing apparatus, a method to simply increase the frequency of clock may be considered. However, in such a method, there is a problem in that sufficient response characteristic between clock for a reading a solid-state image sensor and rotational frequency of a rotational polygon mirror constituting a writing device cannot be obtained due to a structural reason of the rotational polygon mirror so that a device to concurrently output two lines by using two laser diodes has been proposed.
FIG. 8 is a block diagram showing schematic constitution of an image processing apparatus wherein double line writing has been realized. FIG. 9 is a block diagram showing schematic constitution of the image processing circuit shown in FIG. 8. FIG. 10 is a time flow chart showing the relationship between an image processing unit used in a conventional image processing apparatus before and after frequency was converted.
As shown in FIG. 8, due to a structure of the above-mentioned image processing apparatus composed of dual channel registration type CCD (hereunder, simply referred to as "CCD") 1, A/D converters 2 and 3, synthesis circuit 4, image processing circuit 5, frequency conversion circuit 6 and PWM circuit 7 and 8, high speed writing becomes possible.
CCD 1 reads the original image signals of odd numbered pixels (hereunder, referred to as "odd pixel") and even numbered pixels (hereunder, referred to as "even pixel"), and then, send them to A/D converter 2 and 3. The A/D converter provides the original image signals with A/D conversion processing, and then, sends them to synthesis circuit 4. After synthesis circuit 4 synthesizes odd pixel image data and even pixel image data, they are converted to continuous pixel data, and then, they are sent to image processing circuit 5. Image processing circuit 5 conducts several types of necessary image processing, for example, luminance to density conversion, enlargement or reduction processing and spacial filtering so as to send the image data to frequency conversion circuit 6, which is for example, constituted of a spacial filter which converts the spacial frequency characteristic of image information as shown in FIG. 9. In order to conduct a matrix operation of 5.times.5 lines, the spacial filter conducts operations using image information for 5 continuous lines using line memories 11 through 14. Frequency conversion circuit 6 writes image data after subjecting the spacial frequency conversion successively in line memories 21 through 24 for synchronizing with the writing frequency of a writing device. The data are concurrently read in combination of line memories 21 and 23 and line memories 22 and 24, and are then sent to PWM circuits 7 and 8. PWM circuit 7 and 8 independently provide pulse width modulation, and record concurrently for 2 lines by means of double laser diode.
In FIG. 10, CONT with WCLK is a timing signal obtained through a revolving polygonal mirror constituting a writing device synchronously with clock for writing. It also shows one scanning duration for writing one scanning line of about 4700 pixels. CONT with CLK is a timing signal which is synchronous with the clock for image processing. It is a reading timing signal for the image sensor. It indicates reading direction of one scanning line of about 4700 pixels. As described above, image data from the image reading device are ordinarily outputted on a line basis synchronously with a clock. Speed for image-processing the above-mentioned pixel data is determined by the above-mentioned clock. Here, if image data for two lines are recorded concurrently by a double laser diodes, the frequency of WCLK is lower compared to that of CLK. For example, when 25 MHz is selected as the clock frequency for image processing, clocks CLK2 for odd data and even data when A/D are converted are respectively 12.5 MHz. In addition, as a clock WCLK for writing, a clock of about 16 MHz is selected.
To realize a further speed increase, the frequency of clock CLK for image processing must be further raised. The upper limit of the operation speed of image operation elements and that of operation speed of the line memory used for image processing is around 30 MHz in ordinary cases. It is difficult to operate them at higher frequency. In addition, the higher the frequency is, the electro-magnetic interference problem (EMI) becomes more serious.
In addition, the present invention relates to an apparatus in which reading of an original is done in two dimensions by the use of one-dimensional image sensor.
Heretofore, an image reading apparatus obtaining two-dimensional electrical image information by means of the above-mentioned one-dimensional image sensor, wherein the above-mentioned one-dimensional image sensor and the original are relatively shifted to the secondary scanning direction (the direction which crosses the pixel row of the one-dimensional line sensor at a right angle), while the original is scanned in the primary scanning direction by means of the one-dimensional image sensor (line sensor) (see Japanese Patent Publication Open to Public Inspection.
Here, operation of standard one-dimensional CCD sensor (CCD line sensor) will be explained referring to FIGS. 21 and 22.
As shown in FIG. 21, the CCD line sensor is composed of a photo-diode which senses light, shift gates 1 and 2 which shift charge in proportion to the light amount accumulated in aforesaid photo-diode and CCD analog shift registers 1 and 2 for outputting charge serially shifted through the above-mentioned shift gate. Incidentally, in the CCD line sensor shown in FIGS. 21 and 22, effective pixel number is 2048 pixels.
Encircled numerals (1, 2, 3, 4, 5, 6, 19, 20, 21 and 22) respectively in FIGS. 21 represent the CCD pin numbers.
When control signal (shift pulse) SH is at a high level, charge accumulated in the photo-diode is transferred in parallel to CCD analog shift registers 1 and 2 through shift gates 1 and 2. Following this, charge wherein CCD analog shift registers 1 and 2 were read at clock .O slashed.1 and .O slashed.2 for shift are serially shifted and outputted to OS. This OS outputting is image data corresponding to one line.
Namely, outputting for one line is obtained for every one cycle of control signal SH. Cycle (outputting cycle for one line) of the above-mentioned control signal SH necessitates outputting time for one line (the time necessary to serially output for one line) or more.
In order to output all charges transferred from photo-diode of CCD analog shift registers 1 and 2 to OS, in an example of FIGS. 21 and 22, 2124 clocks are necessary as 2124 pixels, i.e., the clock number of .O slashed.1 and .O slashed.2. Time which corresponds to these 2124 clock is time for outputting one line.
Here, the maximum reading speed per line of CCD is determined by the maximum operation frequency of .O slashed.1 and .O slashed.2 and the number of photo-diode. For example, if 5000 pixels are read by one line with operation of 20 MHz, it takes at least 250 .mu.s (in the case of a single channel) for reading one line. These 250 .mu.s are the outputting time for one line. In the above-mentioned example, when the 250 .mu.s is a cycle of control signal SH, it is necessary to set 254 mm/s for the conveyance speed in the secondary scanning direction in order to read the original with 400 dpi in the secondary scanning direction.
As described above, in the case of an image reading apparatus employing a one-dimensional CCD sensor, reading speed can be determined by the operation frequency of the CCD and the number of photo-diodes. In order to increase reading speed, it is necessary to further increase operation frequency. However, since there is a limit to operation frequency, in order to further increase the speed, it is necessary to sacrifice resolution. So far, it was difficult to be compatible high resolution and high speed reading.
As a method to realize high speed reading without sacrificing resolution, as shown in FIG. 23, there is a method to reduce the reading time by the CCD line sensor to 1/4, by arranging 4 CCD line sensor alternately in the primary scanning direction for parallel processing.
However, due to this method, the CCD line sensors are positioned alternately in the primary direction. Accordingly, this resulted in a problem of complicated image processing being necessary for obtaining image data for one line by synthesizing a connection portion between each CCD line sensor without disorder feeling.
In addition, in the case of digital copying machines, enlargement and reduction in the secondary scanning direction may be realized by changing conveyance speed. In such cases, space between each CCD line sensor in the secondary scanning direction is not a constant line number. Accordingly, another problem surfaced in that complicated image processing became necessary in response to magnification (conveyance speed).